"La Ligne de Sceaux" (The Sceaux Line) was a railway line in France running from Paris to some of its southern suburbs, including Sceaux from which it takes its name. It is now part of the RATP's RERline B, though the name is still used informally.

It was originally built with a track gauge of 1,750 mm (5 ft 87⁄8 in) and very tight bends in the area around Sceaux: Arnoux had devised a new articulation system that allowed train wheels to turn, the système ferroviaire dit Arnoux ("Arnoux system"), and the line was to test his prototypes. Shortly after opening, the bogie was invented in the United States, rendering Arnoux's system pointlessly complicated. The section of track from Bourg-La-Reine to Sceaux was rebuilt to the current terminus of Robinson.

For much of its history, the line operated in splendid isolation from other railways. Most notably, the line was extended from its original Denfert terminus further into Paris to a terminus station built underground at Luxembourg between 1889 and 1895. Perhaps because of this deep penetration into the city center, the line was acquired by the Chemin de Fer Métropolitain de Paris (CMP) (see Paris Metro) in 1937.

The CMP immediately proceeded to upgrade its new property, electrifying it with an overhead system down to Saint Rémy-les-Chevreuse in 1938. The outlying section to Limours, where traffic was minimal, was never electrified and closed to passengers on May 15, 1939. From this time on it was generally considered as a metro line, even though it ran on different gauge and with different rolling stock. Maps of the metro network show it along with the numbered lines with an 'S'.

The line was operated as a separate division of the CMP and its descendant RATP, with dedicated rolling stock (other CMP lines used third-rail electrification), until 1977, when it was incorporated into the RER as the southern part of its new line B.